1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) device, and, more particularly, to an organic EL device having an emission layer comprising a polymer and having an improved lifetime characteristic.
2. Description of the Related Art
An electroluminescent (EL) device is a self-emission display using a phenomenon in which, when a current is applied to a fluorescent or phosphorescent organic compound film, light is emitted from the organic compound film by electron-hole recombination occurring. The EL device is lightweight, has non-complex components, and has a simplified manufacturing process, while exhibiting a high visibility and a wide viewing angle. Also, the EL device can present a motion picture display and can achieve a high color purity.
The organic EL device can be divided into two types: a passive matrix (PM) type, and an active matrix (AM) type according to a driving method thereof.
According to a PM drive type, a first set of electrodes and a second set of electrodes are arranged in a matrix configuration to cross each other to produce a pixel area at each intersection. When scan lines are sequentially selected, a pixel area selected by a data line signal instantaneously emits light. In a display apparatus of such a PM drive type, since the configuration and manufacturing process are simple, the manufacturing cost is advantageously low. However, a high resolution and large area display are difficult to achieve, and power consumption is high.
According to an AM drive type, a plurality of thin film transistors (TFTs) and pixel electrodes electrically connected to the TFTs, are arranged at the respective intersections of scan lines and data lines, and common electrodes are entirely covered over the TFTs and pixel electrodes. Since a pixel area is indirectly driven by a TFT as a switching device, voltages applied to the respective pixels are completely independently sustained, and the respective pixels are independently driven according to electrical signals applied to scan lines and data lines. An AM driven organic EL device can achieve a high resolution and large area display and provides a good picture quality while having smaller power consumption and a longer lifetime, compared to a PM driven organic EL device.
An organic EL device can also be divided into a low molecular (or a small molecular) organic EL device and a high molecular (or a polymeric) organic EL device according to a material used for forming an organic compound film.
A low molecular weight organic EL device is basically constructed such that a low molecular weight emission layer is formed between a first electrode and a second electrode, a hole transport layer (HTL) is formed between the emission layer and the first electrode, and an electron transport layer (ETL) is formed between the emission layer and the second electrode, thereby improving efficiency and lifetime characteristics (U.S. Pat. Nos. 4,356,429, 4,539,507, 4,720,432, and 4,769,292).
A high molecular weight organic EL device, as disclosed in U.S. Pat. No. 5,247,190, is constructed such that an emission layer is formed between a transparent, first electrode and a metallic, second electrode. However, such a high molecular weight EL device has poor efficiency and lifetime characteristics because work functions of the first and second electrodes and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) values of a polymer forming the emission layer are different from each other. To solve these problems, there has been proposed a method in which a metal having a work function value similar to the LUMO value of a polymer forming the emission layer is used as a second electrode forming material.
However, if the LUMO value of one selected from red (R), green (G) and blue (B) polymeric emission layers is adjusted to be substantially the same as a work function value of a cathode, the efficiencies of non-selected polymeric emission layers may deteriorate.
For realizing a full-color display using a polymeric organic EL device, an emission layer is formed by ink-jet printing (Japanese Patent Laid-open Publication No. 10-153967, WO 98/24271 and U.S. Pat. No. 6,087,196). The use of ink-jet printing reduces an amount of an emission layer forming material and allows large area patterning.
Another method for realizing a full-color display using a polymeric organic EL device is disclosed in U.S. Pat. No. 5,998,085, in which an emission layer is formed by laser induced thermal imaging (LITI).
As described above, in the polymeric organic EL device, a polymer for forming an emission layer acts as both an electron transport layer and a hole transport layer. Thus, a single emission layer is generally formed between the first and the second electrodes.
However, use of such a single emission layer makes a metal diffuse into the emission layer during deposition of the metal, considerably deteriorating a stability of the EL device (Surface Science 500 (2002) 904–922).
U.S. Pat. No. 5,807,627 discloses a polymeric organic EL device having a four-layered structure in which a first charge carrier injection layer, first and second layers made of a semiconductive conjugated PPV-based polymer, and a second charge carrier injection layer are sequentially stacked between the first and the second electrodes. As described above, a multilayered film that includes a charge carrier injection layer is formed between the first and the second electrodes to increase the efficiency of an organic EL device that is hindered by a polymer's poor capability in transporting electrons and holes.
In the organic EL device, however, to prevent a lower layer from being damaged when forming an upper layer in the manufacture of a multilayered film, polymers having different solubilities in an organic solvent for spin-coating should be used as materials for forming the upper and lower layers. Also, since spin-coating is employed in film formation, patterning of an emission layer is impossible, making full-color display difficult.